Definitions and images to illustrate geological terms, links to images and website articles


diapir bulging upward through overlying strataA diapir or piecement structure results from the upward intrusion of a more buoyant material into/through overlying strata. Diapirs are most commonly composed of evaporitic salt deposits (salt domes) or gas charged muds, but may be igneous.

As ancient seas evaporated they left salt deposits that were buried by sediment. Because the salt deposits were less dense than overlying rock the buoyant mass of salt ballooned upward, intruding into the overlying rocks through weak spots. The intruding “salt bubble” is called a salt diaper, and in most environments, salt diapirs erode rapidly on reaching the surface, leaving craters such as the ones shown below left (Salt Dome & Craters on Melville Island). In arid regions, salt domes may persist (below right - click to enlarge - Salt Dome in the Zagros Mountains, Iran ).

Salt Dome & Craters on Melville IslandSalt Dome in the Zagros Mountains, IranIf the rising plug of salt (called a salt diapir) breaches the surface, it can become a flowing salt glacier (bottom right - click to enlarge image - Iran's salt glaciers).

Iran's salt glaciers

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[links: images: formations: gabbroic anorthosite diapir, west side of Wagers Peak in LZb (postulated mechanisms of formation are 1. that this is a diapir of liquid or mush that pushed aside and broke the overlying layers and then caused turbulence in the magma that caused deposition of the disturbed bed, or 2. that this body is actually a surface deposit of relatively hydrous, plagioclase-rich residual liquid that percolated upward through the crystal mush floor cumulates.)

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